The present invention relates to so-called rapid-focus binoculars in which a knob rotation of less than 45 degrees is sufficient to displace the ocular-lenses through a full focusing range, and more particularly to a focus-adjusting mechanism for reciprocating an ocular-lens bridge member with respect to an objective-lens hinge member upon turning of the knob.
A rapid-focus binocular has been disclosed in U.S. Pat. No. 4,066,329 that has a hollow hinge pin fixedly supported on front and rear brackets of both objective-lens housings, a bridge pin axially movably inserted in the hinge pin for supporting both ocular-lens housings, a knob rotatably mounted on the hinge pin, and a focus-adjusting mechanism for working interconnection between the bridge pin and the knob. The mechanism has a cam-slot provided in the knob, an axial guide slot formed in the hinge pin, and a cam-pin projecting radially from the bridge pin through the guide slot into the cam-slot. The cam-slot slopes so steeply with respect to the rotational axis of the knob as to effect a full-range movement of the bridge pin by a short rotational stroke of the knob.
The mechanism as described above has disadvantages, one of which is that a backlash between the cam-slot and the cam-pin can not be so diminished that the ocular-lens housings are smoothly moved without shaking. The steepness of the cam-slot permits the ocular-lens bridge member to easily move even if a single thrust acts on the ocular-lens housings. This leads to another disadvantage that a focus, once adjusted, is easily upset when the eyepiece is touched to user's face. A further disadvantage is that the mechanism cannot lengthen the optical distance between ocular and objective lenses in accordance with tolerance in each finished binocular without providing an additional insertion of a washer or the like on the bridge pin. The rapid-focus binocular is usually arranged to have a full length of ocular movement longer by 0.5 mm to 1 mm than a necessary length, so that it is needed to modify the outermost position of the ocular-lens housings if the optical distance between the ocular and objective lenses is to be lengthened more than 1 mm from a predetermined one due to tolerance of optical elements such as lenses and prisms in finished binoculars. For example, in the case of 7-power binocular with 50 mm diameter objective lenses, the distance tolerably deviates within a range of +1.3 mm to -2.6 mm from the predetermined one under Japanese Industrial Standard. Accordingly, it is unexceptional to modify the outermost position of the ocular-lens housings relative to the objective-lens housings. It is not only difficult but also awkward to attach a washer on the bridge pin for elongation of the optical distance between ocular and objective lenses in finished binoculars.
It is the primary object of the invention to provide a rapid-focus binocular free from the disadvantages as described above.
The binocular of the invention comprises a hollow hinge pin centrally disposed between front and rear brackets of both objective-lens housings, a peripheral guide slot provided in the hinge pin, a center shaft rotatably fitted in the hinge pin, a knob rotatably mounted on the hinge pin, a connecting pin provided between the knob and the center shaft through the guide slot, an internal multiple helicoid provided in the cylindrical portion of the center shaft for engagement with an external multiple helicoid formed on a bridge pin which is centrally disposed between both ocular-lens housings for integral connection thereof. The internal helicoid is rotated to reciprocate the external helicoid upon turning of the knob. The multiple helicoid mechanism is substantially free from backlash and arranged to provide a smooth ocular movement through a full focusing range with a knob rotation of less than 45 degrees. The internal helicoid is angularly adjustable with respect to the axis of the center shaft and long enough to modify an optical distance between the ocular and objective lenses, if needed.
Other and further objects, features and advantages of the invention appear more fully from the following description.